A commensal symbiotic factor derived from Bacteroides fragilis promotes human CD39⁺Foxp3⁺ T cells and T_reg function

Polysaccharide A (PSA) derived from the human commensal Bacteroides fragilis is a symbiosis factor that stimulates immunologic development within mammalian hosts. PSA rebalances skewed systemic T helper responses and promotes T regulatory cells (T_regs). However, PSA-mediated induction of Foxp3 in humans has not been reported. In mice, PSA-generated Foxp3⁺ T_regs dampen Th17 activity thereby facilitating bacterial intestinal colonization while the increased presence and function of these regulatory cells may guard against pathological organ-specific inflammation in hosts. We herein demonstrate that PSA induces expression of Foxp3 along with CD39 among naïve CD4 T cells in vitro while promoting IL-10 secretion. PSA-activated dendritic cells are essential for the mediation of this regulatory response. When cultured with isolated Foxp3⁺ T_regs, PSA enriched Foxp3 expression, enhanced the frequency of CD39⁺HLA-DR⁺ cells, and increased suppressive function as measured by decreased TNFα expression by LPS-stimulated monocytes. Our findings are the first to demonstrate in vitro induction of human CD4⁺Foxp3⁺ T cells and enhanced suppressive function of circulating Foxp3⁺ T_regs by a human commensal bacterial symbiotic factor. Use of PSA for the treatment of human autoimmune diseases, in particular multiple sclerosis and inflammatory bowel disease, may represent a new paradigm in the approach to treating autoimmune disease.